204 



GREENHOUSES 



boilers in which the burning 'gases travel 

 through tubes or flues which are surrounded 

 by water; and (3) water-tube boilers in 

 which the water is contained in tubes about 

 which the burning gases circulate. Many 



Fig. 103. A type of "vertical ' or 

 "square" sectional boiler 



manufacturers combine two, and sometimes 

 all, of the above types in one boiler. The 

 two latter types are more commonly used 

 for power purposes than is the first, but for 

 heating establishments of moderate size a 

 modification of the first is widely used. 



